While it is thought some of the water was lost to space, research has suggested that is not the full story, and that water could have been incorporated into minerals, buried as ice, or even exist in liquid form deep within the planet’s crust.

Now scientists say their calculations suggest vast quantities of liquid water are to be found trapped within rocks about 11.5-20km below the Martian surface.

“Our liquid water estimate is more than the water volumes proposed to have filled possible ancient Martian oceans,” said Dr Vashan Wright, a co-author of the study from Scripps Institution of Oceanography at the University of California San Diego.

Writing in the Proceedings of the National Academy of Sciences, Wright and colleagues report how they made calculations based on gravity data for Mars and measurements recorded by Nasa’s InSight lander. The latter reveal how the speed of seismic waves – created by Marsquakes and meteorite impacts – change with depth inside the crust of the red planet.

“A mid-crust whose rocks are cracked and filled with liquid water best explains both seismic and gravity data,” Wright said.

Wright added that if the measurements at the Insight lander location were representative of the whole planet, the amount of water trapped in the rock fractures would fill a 1-2km-deep ocean on Mars.